Studies were carried out to understand cellular targets for in vivo transformation by two clonal replication competent retroviruses, Moloney- and Rauscher-MuLV. Moloney-MuLV was found to induce T-cell tumors in several strains of mice tested, while Rauscher-MuLV produced tumors of B-cell origin in the same strains of mice. The majority of Rauscher-MuLV lymphoma cell lines expressed immunoglobulin heavy (Mu) chain in the absence of detectable light (Kappa or Lambda) chains. However, 10-20% of the tumors were comprised of immature cells making no Ig-related products or more differentiated cells producing both Mu and Kappa chains. Studies on the structure of Ig-related genes revealed the rearrangement of Mu-genes in essentially all cell lines, irrespective of Mu-synthesis, while rearrangement of Kappa-genes was restricted only to producer cell lines.